Elektronenspektren der oven-shell-Svsteme

Ohne Zusammenfassung     

Berechnung der Elektronenspektren der Isocyanatoligomeren

Ohne Zusammenfassung     

CNDO/2-Untersuchung der Dimerisation des Difluoraminoradikals

Ohne Zusammenfassung     

Elektronenspektren von 1,1-Diphenyl?thylen und Benzophenon

Ohne Zusammenfassung     

PPP-Berechnungen und Vergleich der Elektronenspektren von iso-π-elektronischen Sauerstoff-, Stickstoff-, Schwefel- und Selenheterocyclen

Zusammenfassung 42 Heterocyclen, die sich formal von alternierenden und nichtalternierenden Kohlenwasserstoffen durch Ersatz einer Doppelbindung durch die Heteroatome Sauerstoff, Stickstoff, Schwefel und Selen ableiten, werden nach dem Pariser-Parr-Pople-Verfahren berechnet und ihre Spektren im nahen Ultraviolett und Sichtbaren beschrieben. Schwefel und Selen werden dabei im LCAO-MO-Verfahren nach dem p-Modell behandelt. In den meisten Fällen können die Berechnungen nicht nur die spektralen Charakteristika gut wiedergeben, sondern auch Beziehungen in den Spektren iso--elektronischer Verbindungen gleicher Struktur verständlich machen. Berechnungen an kationischen Stickstoffheterocyclen führten dann zu falschen Ergebnissen, wenn die Stickstoffparameter vom Pyrroltyp übernommen werden.

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PPP-calculations and comparison of electronic spectra of iso--electronic oxygen, nitrogen, sulfur, and selenium heterocycles

42 heterocyclic compounds, formally derived from alternant or nonalternant hydrocarbons by replacing a C=C group by an oxygen, nitrogen, sulfur or selenium atom, are studied by the Pariser-Parr-Pople-type of calculation. By means of these results the near ultraviolet and visible spectra are described. The sulfur and the selenium atoms are treated in the LCAO-MO-method using the p-model. In most cases spectral features are not only well reproduced, but interrelations of the spectra of iso--electronic compounds, possessing analogous structures are interpreted, too. On calculating cationic nitrogen heterocycles using parameters of pyrrol type nitrogen results are unsatisfactory.

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Résumé 42 composés hétérocycliques, formellement dérivés d'hydrocarbures alternants ou non alternants par remplacement d'un groupement C=C par un atome d'oxygène, d'azote, de soufre ou de sélénium, sont étudiés par la méthode de Pariser-Parr-Pople. Les résultats obtenus permettent de décrire le spectre U.V. proche et visible. Soufre et sélénium sont traités en utilisant le modèle p. Dans la plupart des cas les caractères spectraux sont non seulement correctement reproduits, mais les relations entre les spectres des composés iso--électroniques à structures analogues peuvent être interprétées. Les calculs concernant les heterocycles à azote cationique sont insatisfaisants lorsqu'ils sont effectués avec des paramètres de l'azote du type pyrrole.

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Anmerkung: Die Arbeit wurde in dem von Prof. Dr. R. Mayer geleiteten Schwefelarbeitskreis der TU Dresden durchgeführt.     

Freie Elektronenpaare in lokalisierten CNDO/2-Wellenfunktionen der Stickstoffverbindungen

The method for constructing hybrid orbitals described earlier is now used for the analysis of CNDO/2 wave functions of molecules containing nitrogen atoms in order to get information on the type of hybridization of nitrogen lone pair orbitals in various compounds.     

“Universal” CNDO/2 method (CNDO/2-U)

The Pople-Santry-Segal CNDO/2 method and the modified CNDO/2 version worked out recently by Clack for transition-metal complexes have been extended to the full periodic table.     

Quantenchemische modellrechnungen zur ionischen polymerisation mit hilfe der EHT- und CNDO/2-Methode

Both the EHT and the CNDO/2 methods were applied to the calculation of simple models of the active centres in ionic polymerization. For the models of active anionic centres CH₃^?Mt^? and CH₃O^?Mt⁺ (Mt = Li, Na, MgX), only the results obtained by CNDO/2 permitted conclusions on comparative reactivities; the EHT method failed.Both methods gave results of good agreement for the models of active cationic centres of the type R⁺MtX_{n + 3}^? (R = H, CH₃, OH₃; Mt = B, Al; X = F, Cl). The counterion remains in its tetrahedral structure during the interaction with the cation.     

Schwingungs- und Elektronenspektren der bindungsisomeren Hexakis(thiocyanato-isothiocyanato)-ruthenate(III)

Vibrational and Electronic Spectra of Bond-Isomeric Hexakis(thiocyanato-isothiocyanato)ruthenates(III) Well resolved IR, Raman, and Electronic spectra of the bond isomeric complexes (TBA)₃[Ru(NCS)_n(SCN)_6?n], n = 1–5, including the pairs of geometric isomers n = 2, 3, 4, are recorded at low temperatures (10 and 80 K). Characteristic vibrations of the N- or S-coordinated ambident ligand SCN^? occur as listed: ν_CS(N): 810–850, ν_CS(S): 690–710, δ_NCS: 450–490, δ_SCN: 420–450, ν_RuN: 300–350, ν_RuS: 270–295 cm^?1. The assignment of the complexes is based on stepwise increasing intensities of the ν_CS(N) modes with increasing number of N-coordinated ligands. Characteristic shifts and splittings in the spectra allow to distinguish the geometric bond isomers according to their different symmetries. Even the absorption spectra in the visible range show within the series of bond isomers and for the cis/trans pairs systematic alternations.     

Identifizierung von neuen Chrom (III) -nitritokomplexen mit Hilfe von Elektronenspektren des Nitritliganden in dipolaren L?sungsmitteln

Ohne Zusammenfassung     

Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

CNDO/2 calculations of the interactions of formamide and imidazole: Another failure of CNDO/2

The interaction energy curve of the formamide-imidazole system resulting from the CNDO/2 method exhibits a superfluous minimum at a much smaller separation than the sum of the van der Waals radii of the closest atoms, and a low maximum at this latter distance. The relationship of the interaction energies using CNDO/2 and those using the atomic charges on the two molecules is investigated for this system.     

CNDO/2 calculations of some polyazaindenes

The theoretically estimated dipole moments of indolizine, imidazo[1,2-a]pyridine, imidazo-[1,5-a]pyridine and of pyrazolo[ 1,5-a]pyridine obtained by the CNDO/2 approximation have been compared with the experimental values. The bond angles and bond distances for these polyazaindenes have been estimated.     

A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

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Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

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Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

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This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

CNDO/2 study of benzene-chlorine complex

We have carried out CNDO/2 calculations for the benzene-chlorine complex in a number of configurations. In order to obtain reasonable complexation energies, it was necessary to exclude chlorine 3d-orbitals from the basis set. When onlys- andp-orbitals were included, we found maximum stability for the complex for an unsymmetrical geometry. The calculated complexation energy was ?3.5 kcal/mole. Configuration-interaction calculations predicted a wavelength of about 270 nm for the charge-transfer band. Most, but not all, geometries were found to produce stable complexes.     

Zur theoretischen Interpretation von NMR-chemischen Verschiebungen tetrakoordinierter zentralatome. IV—CNDO/2-Berechnungen von 29Si-NMR-chemischen Verschiebungen. Der Einfluß der ΔE-Näherung

On the basis of the CNDO/2 method paramagnetic screening constants of the central atom of tetrasubstituted silicon compounds of the type Me_4–nSiX_n (X = F, OMe, NMe₂, C1) are calculated, both with and without ΔE approximation. The results are compared with the experimental ²⁹Si n.m.r. chemical shifts. The ‘averaged excitation energies’ ΔE obtained from the comparison of calculated values depend on the charge of the central atom and cannot be considered to be constant for quantitative studies.     

Eine Methode zur Untersuchung der Zusammensetzung von Aluminiumlegierungen auf Grund der Verwendung von pH-Indikatoren

Ohne Zusammenfassung     

Relationship between EHT and CNDO/2 calculations

Multiplying EHT atomic charges and dipole moments by 0.3 gives approximate CNDO/2 values.Financial support from a CPGA is gratefully acknowledged.